While our current understanding of the pathophysiological mechanisms involved in the development of ethanol (EtOH)-induced liver injury is incomplete, both innate and adaptive immune responses are implicated in initiation and progression of liver injury. The complement (C) system is a network of more than 30 proteins, involved in both innate and adaptive immune responses. Activation of the C pathway can occur via the classical, lectin or alternative pathways, all three pathways culminate in the activation of C3, which in turn activates the terminal pathway leading to the formation of the C5b-9 membrane attack complex (MAC). C is critical to an organism's ability to ward off infection and also plays a role in the repair response to injury, but uncontrolled C activation may itself cause cellular injury. Further, the C pathway contributes to activation of a number of pro-inflammatory responses. We have recently investigated whether C activation is required for chronic EtOH-induced liver injury in wild-type mice and mice lacking the 3rd (C3) or 5th (C5) components of the C activation pathway, as well as mice lacking decay accelerating factor (CD55/DAF) or CD59, intrinsic regulatory proteins in the C pathway. Chronic EtOH feeding to mice increased activation of C3, increasing the concentration of C3a in the plasma. Importantly, C3 -/- and C5 -/- mice were protected from EtOH-induced fatty liver injury. Injury was exacerbated in CD55/DAF -/- mice, while CD59 -/- were protected from EtOH-induced fatty liver injury. Here we propose to test the hypothesis that C activation is critically involved in the development of chronic EtOH-induced fatty liver and that the intrinsic C regulator CD55/DAF acts as a primary brake against the deleterious effects of EtOH in the liver. Further, we hypothesize that activation of the terminal C pathway and formation of the MAC serves to dampen EtOH- induced injury in the liver. We propose four specific aims to investigate the mechanisms by which the C system modulates EtOH-induced liver injury in a mouse model of chronic EtOH exposure, as well as in primary cell cultures. 1) Elucidate the specific C activation pathway(s) involved in activation of C3- dependent damage by EtOH, 2) Determine the specific contributions of C3 versus C5 in mediating chronic EtOH-induced fatty liver injury, 3) Understand the protective function of the terminal C pathway/MAC during chronic EtOH exposure and 4) Determine the effects of chronic EtOH feeding on the response of Kupffer cells and hepatocytes to activation by C3a and C5a. Understanding the mechanisms by which chronic EtOH feeding activates the C system and the role of C in mediating liver injury will facilitate the development of pharmacotherapeutic strategies to prevent and/or reverse EtOH-induced liver injury.